This invention relates to a method for producing a porous SiO2 xerogel with a characteristic pore size of less than 1 micrometer using temporary organic solid skeletal supports, which are removed by thermal oxidation at the end of the production process by means of a sol-gel process with subcritical drying of the gel. The additional organic particles, or macromolecules, in the inorganic gel prevent a collapse of the inorganic network during the subcritical drying process. These organic solid skeletal supports are subsequently oxidatively removed as far as possible by heat treatment in excess of 300° C. The result is a SiO2 xerogel (having a fiber content of <5% by weight) with a porosity >80%, a carbon content of less than 10%, which is not or is only weakly chemically bound to the silicate structure, and pores in the range of less than 1 micrometer.
Aerogels, cryogels and xerogels are used in many areas. The aforementioned materials are basically differentiated by the type of drying method. Aerogel is a generic term for all gels having a low solids content, the pores of which are filled with air, but in a narrower sense they are also defined by a supercritical drying process, cryogels by freeze drying and xerogels by convective subcritical drying.
With regard to the present aerogel according to the invention, this is therefore strictly speaking a xerogel throughout.
As a result of their extremely low densities and their high porosities of typically 85% and greater, silica aerogels are excellent insulating materials which in contrast to organic materials can also be used at high temperatures. In the case of non-evacuated materials, above 250° C. organic components would combust with the oxygen present in the air.
The production of highly porous solids by means of sol-gel methods normally requires a supercritical drying step in order to obtain the pore structure. This drying is on the one hand demanding in terms of time and resources because as a general rule the solvent in the pores first needs to be exchanged. On the other hand, it is energy intensive because autoclaves operate at high pressure. Processing in an autoclave is also disadvantageous from the process engineering viewpoint on account of its non-continuous nature (batch processing). Due to the great capillary forces occurring, convective drying at 1 bar (subcritical drying) results in the collapse of the pore structure, which is why monolithic materials having a high porosity can only be produced with difficulty. This means that xerogels exhibit higher densities and therefore also inferior thermal insulation properties compared with aerogels.
The aerogel produced in [WO2005068361] must be dried supercritically and is thus expensive and complex to produce.
In order to avoid the supercritical drying and nevertheless achieve low densities there are several general approaches. Einarsrud et al. have developed a method which reduces the shrinkage occurring during subcritical drying through a stiffening of the gel structure in the wet gel [Einarsrud, M. A., E. Nilsen, A. Rigacci, G. M. Pajonk, S. Buathier, D. Valette, M. Durant, B. Chevalier, P. Nitz, and F. Ehrburger-Dolle, Strengthening of silica gels and aerogels by washing and aging processes. Journal of Non-Crystalline Solids, 285 (2001) 1-7]. However, in spite of the low density of the resulting xerogel the solid-body heat conduction is increased when using this method because locally the contacts between the silica particles in the gel structure are systematically increased.
A further disadvantage are the additionally required method steps which comprise two time-consuming solvent exchange steps and the long times which are required for crack-free drying of macroscopic molded bodies.
In order to prevent a cross-linking of the surface hydroxy groups (in the case of compression of the gel by the capillary forces) during drying and thus an irreversible shrinkage, these groups can be converted using a silylating agent as in [EP0690023A2], WO9805591A1 or WO96022942A1. This method does however always signify a further protracted solvent exchange and a further synthesis step and does not prevent the large temporary shrinkage of the sample during the subcritical drying which particularly in the case of molded bodies having dimensions in the cm range and greater can easily result in the formation of cracks or requires very slow drying. These silylated hydrophobized gels cannot be used with application temperatures in excess of 250° C. because otherwise the organic surface groups are destroyed and the desired effect, such as for example the hydrophobia in the case of WO9805591A1, is thus also destroyed.